1. Field of the Invention
This invention relates generally to so-called silk screen printing equipment and more particularly to a novel reinforced screen printing frame primarily intended for very large area printing applications.
2. Reference to Copending Applications
Reference is made to my copending application Ser. No. 07/576,673, filed Aug. 31, 1990.
3. Prior Art
The art of screen printing, commonly referred to as silk screen printing, is very well known and extensively used for a vast assortment of printing applications and hence need not be elaborately explained in this disclosure. Suffice it to say that screen printing involves the use of an open rectangular screen frame across one side of which is placed a printing screen having a pattern of open and blocked holes corresponding to the image to be printed. Along at least two opposite sides, and often along all four sides, of the screen frame are screen gripping and tensioning means, such as rollers, for gripping the screen edges and stretching the screen edgewise to a taut condition suitable for screen printing.
When printing a work surface, the screen frame is positioned so that the screen is located at the underside of the frame facing the work surface. A printing ink is spread across the upper side of the screen and is forced through the open holes in the screen onto the surface. This action depresses a major central portion of the screen into firm flat contact with the work surface as is essential to produce an acceptable screen print. The entire image formed on the screen is located within this central screen portion, and only this central screen portion is useable for screen printing. The attachment of the screen edges to the frame keeps the remaining border-like portion of the screen between its central useable portion and the gripped edges of the screen from flat contact with the work surface. Accordingly, this border portion of the screen, which may be on the order of 3 inches in width for a 20 inch frame opening, is unuseable for printing purposes. Many screen printing operations involve the sequential use of different printing screens and/or different color inks to produce a finished print.
The present invention is concerned with printing relatively large surface areas, such as areas on the order of 50 feet or more in length and several feet in width. One such large surface area printing application, for example, involves the printing of certain designs, indicia, and the like on a facing sheet for the interior of a Boeing airliner. In the past, it has been necessary to print these designs, indicia, etc. on several separate sheets which then must be applied in proper alignment to the airliner interior. Simultaneously printing of all the required designs and indicia on a single sheet in a single printing operation and application of this single sheet to the airliner is substantially more economical and results in a vastly superior airliner interior.
Up to the present time, it has been either impractical or totally impossible to make such large surface area screen prints. This is due to the fact that the existing screen frame structures are not sufficiently rigid to enable their construction in the large sizes necessary for large surface printing applications. Thus, the existing screen frame structures, if enlarged sufficiently for the large surface area screen printing applications contemplated in this invention, are subject to two modes of bending which seriously degrade the screen print quality. One bending mode is gravity-induced vertical bending or saging which occurs in a large frame of conventional construction when it is raised and lowered in the course of screen printing operations. Such a conventional printing frame on the order of 50 feet or more in length, for example, would sag several inches vertically at its center when lifted. The second bending mode to which a large screen frame of conventional construction is subject is inward bending of its long frame members in the plane of the frame by the tension forces in the stretched printing screen. Both of these modes of bending or deflection create non-uniform tension in the printing screen and thereby severe distortions in the resulting print.